In order to cope with communication traffic that tends to drastically increase in accordance with recent increasing demand for the Internet, a technological development aiming at high speed and large-volume transmission has been advancing. In a submarine cable system supporting international communications, etc., the transmission quality is deteriorated and bit errors are prone to be generated by such as wavelength dispersion due to transmitting a large-volume of data at a high speed for a long distance. Therefore, data is usually transmitted with high efficient error correction code “FEC” (forward error correction). Although, a hard decision error correction method, in which the identification of received data is executed by one decider, has been used as an error correction method for FEC, in recent years, a soft decision error correction method, in which the identification of received data is executed by a plurality of deciders by using a plurality of identification thresholds, has been under investigation in order to improve the error correction capability. (For example, refer to Japanese Laid-Open Open Patent Publications 2002-330104, Page 5 through 7, FIG. 1).
However, there have been problems in that the soft decision error correction capability of the optical signal receiving equipment differs in accordance with difference attributable to manufacturing (hereinafter referred to as manufacturing difference) in individual deciders or deterioration over time thereof, because the identification is executed by the plurality of deciders in conventional optical signal receiving equipment.
Moreover, there have been problems in that electric power consumption is increased in accordance with increasing the volume of circuits, because the identification is executed by the plurality of deciders.
Furthermore, there have been problems in that the error correction capability fluctuates in accordance with amplitude variations of input electronic signals during operations.
The present invention has been made in order to solve the above problems in the conventional optical signal receiving equipment.